3
\$\begingroup\$

If I remove the plate of a wall outlet, and touch the hot screw, I’ll get a shock. The potential difference with the ground is enough to send some current into my hand. Importantly, this happens even if I didn’t disconnect the neutral, which has much lower resistance.

So, by the same logic, why isn’t there current flowing down into the grounding rod of my house? The neutral and ground buses are jumpered at my panel. To the electricity flowing down the neutral at my panel, why isn’t the path to ground offered by the grounding rod at least as “attractive” as the path offered by my body was to the electricity in the outlet?

I’ve seen discussions where people said that this is because the earth has much more resistance than the neutral. But surely, so does my body compared to a hot screw.

Just wondering about this.

EDIT: Just had a thought. Is the difference that at the outlet, hot brings power in and neutral carries it out, whereas at the panel (at least in the US) you have two phases, one bringing power in and the other one carrying it away (back to the pole) as opposed to the neutral? I guess this would make the two situations not identical. Thx.

EDIT 2: I appreciate everyone’s help. I drew a picture of what I’m confused by. It’s not a circuit diagram, just a picture of how the wires are connected in my house. The three vertical lines are the split phase from the utility. My question is: in the pole - L1 - N - pole circuit, what difference is there between a finger added in the left situation, and a finder added on the right? Both lead to the same ground, from the same circuit. Why doesn’t the person on the right get zapped?

illustration

\$\endgroup\$
9
  • \$\begingroup\$ You might want to draw a diagram and include the ‘pole pig’ distribution transformer. \$\endgroup\$ Dec 3, 2021 at 12:25
  • \$\begingroup\$ What earthing system do you have? Groundings rods are for TT and PE/N jumpered at the panel is TN-C-S. Also, what kind of grid, split phase or 3 phase? \$\endgroup\$
    – Rakward
    Dec 3, 2021 at 13:00
  • 1
    \$\begingroup\$ This should be migrated to Home Improvement. \$\endgroup\$
    – SteveSh
    Dec 3, 2021 at 13:00
  • \$\begingroup\$ When you touch the hot/black screw on a switch or outlet, you are putting your body in parallel with any other load on that circuit. So your body has 120V between it and ground. \$\endgroup\$
    – SteveSh
    Dec 3, 2021 at 13:04
  • 4
    \$\begingroup\$ Sorry if this is the wrong community. I find that answers on Home Improvement and similar are usually not very scientific, so I thought I’d try here. I have a typical residential split phase system, US. \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 13:04

8 Answers 8

5
\$\begingroup\$

The ground rod and the neutral returns back to the pole are in parallel. So return current will flow through whatever path has the lowest resistance (inversely proportional to that resistance). The neutral resistance should be a lot lower than the path through the ground rod.

Note that under normal conditions, the ground path should not carry any current. All the current used by a load should be returned via the neutral. The ground rod path is considered a safety ground, and is sometimes referred to as earth ground.

When you touch the black wire or terminal, you are putting a load - your body - across the 120V. That is, between the 120V hot and ground. This is why you experiance a shock. Your body is not in parallel with the neutral or (safety) ground paths.

\$\endgroup\$
10
  • \$\begingroup\$ Thanks. But can you amplify on why the neutral and ground rod wire are in parallel? The neutral leads back to the pole, while the ground goes to (the actual) Earth. If I imagine a simplified scenario of a circuit: pole to panel to outlet to panel to pole, then why would putting a path to ground at the outlet be different than a path to ground at the panel? It’s a path to the (same) Earth, inserted into the same circuit. I know I’m confused about something, so thanks for the effort! \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 13:15
  • 1
    \$\begingroup\$ @Skeptic There's a ground rod at the house, connected to neutral. There's also another ground rod at the transformer, also connected to neutral. So the ground and the neutral are in parallel. In the house, the neutral-ground link must be before any GFCIs, or they will keep tripping. \$\endgroup\$
    – Simon B
    Dec 3, 2021 at 13:22
  • \$\begingroup\$ Sorry to be dense. Wouldn’t what you are saying mean that the ground wire from the panel the pole is in parallel to the neutral, but not the ground wire from the panel to the grounding rod in my yard? It’s the latter I mean. How is that wire, reaching into the circuit, any different from my hand at the outlet (which is part of the same overall circuit)? \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 13:28
  • \$\begingroup\$ Stating that there should not be any current flowing in the ground is a major simplification, possibly confusing OP. Return current will not only flow in the lowest resistance path, it will flow in both. Only that the majority of the current will flow in the lowest resistance path. \$\endgroup\$
    – Klas-Kenny
    Dec 3, 2021 at 13:32
  • \$\begingroup\$ The entire earth/safety ground return path is in parallel with the neutral. The ground bus in your electrical panel -> ground rod -> earth -> pole ground rod -> transformer, all are part of that ground/safety path back to the pole. \$\endgroup\$
    – SteveSh
    Dec 3, 2021 at 13:34
4
\$\begingroup\$

The neutral is the return path for the two 120 V circuits. It's not so for the 240 V circuit.

The electrocution current passes through the body and earth on account of the return path through the grounded neutral.

enter image description here

\$\endgroup\$
9
  • \$\begingroup\$ Ah, thanks. That makes sense. But then I’m all the more puzzled as to why a wire, tired to neutrals at the panel, and connected to a ground rod (which should provide less resistance than my foot) doesn’t experience significant current flow? \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 14:55
  • 1
    \$\begingroup\$ Oh. Or does it? And just that touching the ground rod doesn’t shock me because all I’m doing is providing a parallel connection to the same ground? I guess that’s different than touching the hot… am I on the right track? \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 14:59
  • \$\begingroup\$ No current flows from neutral to ground in the absence of a closed circuit path. Electrocution is caused by current passing from the line through the body, earth, neutral and back. \$\endgroup\$
    – vu2nan
    Dec 3, 2021 at 16:42
  • \$\begingroup\$ Interesting. So are you saying the difference is that current will travel through the finger in both directions, whereas through the ground rod connecting wire it cannot? \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 17:03
  • 3
    \$\begingroup\$ The current (@Skeptic) flows through your finger to ground below your feet then back through ground to the earth rod then back up to where the earth wire bonds to the neutral wire. Current flowing down a wire isn't going to give you a shock if that wire is bonded to ground. \$\endgroup\$
    – Andy aka
    Dec 3, 2021 at 17:27
3
\$\begingroup\$

Current wants to return to source not ground.

This is a common misconception. If a transformer makes hot/live and neutral, the hot/live couldn't care less about going to ground. It wants to go to neutral.

The only reason hot has any interest in going to ground is that we also ran a ground spike to neutral. So it just sees ground as a route to get to neutral via the ground spike we added. Some systems improve safety by not doing that, but making that work is tricky.

Neutral doesn't care about going to neutral. It's already there.

Neutral and ground in North American/NEC installations

USA grounding does not correspond to any European earthing code (the thing with the letters).

Neutral is created at the utility's transformer. Then, it is pegged to a grounding rod at the transformer itself.

2-3 phases and neutral are then delivered to the customer at their electric meter. Ground/earth is not delivered. Neutral is not ground.

Immediately after the electric meter, the customer establishes their own earthing/grounding as the Grounding Electrode System. This is distributed throughout the house on the earth/ground wires in every circuit.

At this service point, there is also a neutral-ground equipotential bond which further pegs neutral to earth voltage (or vice versa, depending on the forces involved).

You say "well, that makes TWO neutral-ground bonds, one at the house and one at the transformer." More like ten - since all the other houses on the block have bonds too.

Normal and fault current paths

Normally, all service current flows among phases and neutral. There should be no current flow on any ground/earth wire at any point, nor on the equipotential bond.

During a fault condition, current flows from phases or neutral to something else. If it is the Equipment Grounding Conductor, that current is efficiently returned to the main panel, through the N-G equipotential bond, to neutral and back to source. If this is a bolted hot-ground fault, it will flow so much current as to instantly “magnetic-trip” the circuit breaker. Neutral-ground faults are undetected, except by GFCI/RCD and AFCI.

If current leaks not to the grounding conductor but to somewhere else like actual earth, then it leaks through the soil to all Grounding Electrodes in the system, back to the ground bar, through the N-G equipotential bond and back to source. Soil is a very poor conductor. The high impedance of dirt will not allow enough current to operate an overcurrent device (plain breaker) in any mode, but it can flow enough (20ma) to kill you. (note that if you're on a roof or in water, a stun is a kill).

If GFCI/RCD protection is installed on that circuit, it will probably flow the 5ma needed for detection and trip it, which will end your shock experience before you enter the "stun" regime.

Sensitivity is that good because NEC applies GFCI/RCD protection on a per-circuit basis. A single circuit is unlikely to have 5ma of ordinary leakage.

Only voltage differentials shock you.

Social Security legislation is often called "the third rail of politics" — touch it and you die.

Electricity isn't like that. To get hurt you must flow current, and to flow current you must contact two things at different voltages. And the voltages must be different enough to push through your body.

Your body is like a VBO - voltage breakover device. Below a certain voltage, your body will not conduct electricity at all.

That makes the neutral and ground wires pretty safe on a working system. Between them and earth, there isn't enough voltage - only a few volts at most - to do you any harm.

That is why your second hand doesn't get shocked from touching the Grounding Electrode System.

\$\endgroup\$
2
\$\begingroup\$

You say that you have a split phase system, ground/neutral jumpered in your panel and a ground rod, so your system looks like this:

enter image description here

I don't understand why you have a ground rod + PE/N jumpered in your local panel? That's mixing TT and TN-C-S.

Grounding rod is supposed to be for TT-networks

enter image description here In that case no current will flow under normal conditions through PE because no load is connected between L/PE. A load between L-N will simply have a return-path through N. N and PE aren't combined till they are the the supply-transformer.

If your neutral/ground are jumpered in your electrical panel, that’s a TN-C-S network.

enter image description here

Current will flow through the N up the jumpered point in your panel, where PE/N are combined into a single wire to the supply transformer. There's no reason for current to go through PE, because PE isn't connected to a ground rod at the consumer's house.

Your situation creates 2 return paths for PE, through the ground-rod and the combined PEN-conductor, but the PEN-conductor has much lower resistance.

\$\endgroup\$
6
  • 1
    \$\begingroup\$ Wait a minute. Aren't neutral and ground tied together at the main panel? But are kept separate in any sub panels? \$\endgroup\$
    – SteveSh
    Dec 3, 2021 at 13:39
  • \$\begingroup\$ Yes, they should be tied at main and not any subpanel. \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 13:42
  • \$\begingroup\$ @Rakward this is awesome. Thanks! I’m not an electrician, just a curious soul. My understanding was that the picture you drew (well, sans the body - haha) is how most North American residential services were constructed, assuming the rod connection is at the panel. (Maybe I misunderstood the diagram?) \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 13:44
  • \$\begingroup\$ There are no main/subpanels neccesary to understand the problem. The black bar in the first drawing is simply the supply-transformer. It's about connection points. But I'm from Europe and have noticed before that US/Europe has difficulty communicating about electrical matters because of different rules/systems. It would help if you drew your layout. \$\endgroup\$
    – Rakward
    Dec 3, 2021 at 13:48
  • \$\begingroup\$ Thanks @Rakward. I think your picture is accurate, except the ground rod is on the consumer side, not the supplier. In a US residential breaker box, the neutral bus bonded to a grounding rod which is buried outside the consumer’s home. \$\endgroup\$
    – Skeptic
    Dec 3, 2021 at 14:57
2
\$\begingroup\$

This is another way for leakage currents to be felt by humans , designed to reduce EMI yet be safe. Example of a portable SMPS charger to a metal case laptop. There are 3 grounds shown. Each is 0V at that location by definition, even if floating on laptop, that is until a VGA cable is connected to a PE protected monitor on a 3 pin plug. Then it is no longer floating.

schematic

simulate this circuit – Schematic created using CircuitLab

Notice there is no premise ground rod to Neutral in home here just the Earth gnd wire. Just PE at Dist. XFMR.

\$\endgroup\$
2
\$\begingroup\$

Very Good Question.
Not many explains why, but accept it as what it is. So thus most of the house inspectors do.

If I remove the plate of a wall outlet, and touch the hot screw, I’ll get a shock.

As you would agree, when you touch HOT, the current flows from hot to the GROUND through your body. When your body is floating, HOT does not get you a shock. You would be shocked when you see some people intentionally touch HOT and not getting shocked. That is one way to view a lineman can handles high power live lines.

The potential difference with the ground is enough to send some current into my hand. Importantly, this happens even if I didn’t disconnect the neutral, which has much lower resistance. So, by the same logic, why isn’t there current flowing down into the grounding rod of my house?

Never try this: If you remove the jumper between the NEUTRAL and the GROUND at the service entrance point, HOT & GROUND/GROUNDING-ROD will supply power, though the voltage and amperage are questionable. Go far back to when my town just got electricity, kids found free electricity, bypassing the meter (those days meter), by taking HOT and something plugged in the dirt, stronger than the HOT and NEUTRAL. We played melting and bending glass pieces using arc generated with lead from pencil. h h..
So, for an example, the current flows to the grounding rod, and get onto another grounding rod nearby then returns to the NEUTRAL. Not only that, it might find higher potential circuit around. GROUNDING points are everywhere, not only at the service drop.

The neutral and ground buses are jumpered at my panel. To the electricity flowing down the neutral at my panel, why isn’t the path to ground offered by the grounding rod at least as “attractive” as the path offered by my body was to the electricity in the outlet?

Yes, as you said, "people said that this is because the earth has much more resistance than the neutral", is one reason why. The resistance means loss and potential difference. Meantime, the GROUND, earth-ground through the grounding rod, is not a single current path for your service/circuit. Many artificial and natural sources, and loads, are on the ground (multiple circuits). Thus many factors are unpredictable.

Why GROUND WIRE (not the earth ground or grounding rod) is not offered as mains.

Ground wire (green wire) is for SAFETY (discussed down there). Current flow through ground is GROUND FAULT, detected and interrupted by GFC, if you have any. GFC monitors if any current flows other than HOT and NEUTRAL, the source and return. So, GFC finds someone got attracted by the beauty of grounding rod, and prevents abusing it.

Your question, now, is:

Why is the grounding rod and the jumper needed?
Why the GROUND-ROD/GROUNDING is needed at all. How is it safe?
If the power plant, or the the distribution transformer at the service drop, had NEUTRAL to the EARTH-GROUND connection already, do we still need grounding rod?

The answer is:
However amount it can be, "current flow" occurs due to potential difference, and the flowing current develops potential difference.

-. When you touch the line, your body potential is REFERENCED on the GROUND (floor, earth ground). If you get electrocuted from hair dryer in the tub, GFC quickly determine you are in danger, since it detected the current flew from HOT to other than the NEUTRAL.

-. If the line gets floated (the NEUTRAL does not get GROUNDED), then the line can be at any potential from the GROUND. That can be really dangerous. Meantime, if you have your own circuit after isolated, like a transformer, generator, solar panel and etc. you still need put that to be based on the GROUND by connecting/coupling (phase isolation) it to the earth ground.

-. The potential develops along the transmission line. Thus, even if the NEUTRAL at source side was grounded, the NEUTRAL at service drop is (possibly) not at ground level. It needs assurance by grounding again at the service drop.

-. If the ground wire takes current (share the return current with NEUTRAL) then the GROUND is not the same potential level as the earth-ground, where your safety is referencing.

-. If the ground wire (that one with ground-rode) shares the return current, without GFC, and gets interrupted (cut off), the current finds undesirable path, like kitchen sink, or even can heat up to fire.

Meantime, utilities and electrical/electronic equipment can leak current, due to imperfect dielectric, or possibly EMI (not the radio frequency), to any exposed electrically conducting surface. When the ground wire is connected, the leakage can be taken to the ground, not to human. That is the purpose of the ground wire attached to washer/dryer and ovens.

\$\endgroup\$
2
\$\begingroup\$

It's the electric current (not voltage) that "hurts", i.e. what disturbs your nervous system. Then again, to cause current to flow, you do need voltage, in the right proportion to the resistance posed by the path...

AC current can flow though capacitive coupling = even without direct galvanic/resistive interconnection. The human body is reported to exhibit a capacitance of about 100-200 pF against a "far ground", which isn't very much. To achieve 1 mA (which is well above the level where you start to notice the current) from a 50 Hz source at 240V AC you need capacitance of about 13 nF.

The ground under your feet, and various metal objects in the building around you, are more or less well connected to the ground potential. The only thing that's not grounded are the live wires. Overall, inside a building or walking outside, your body will have some level of coupling, resistive or capacitive (or a mix of both) to the ground potential.

Thus, if you touch the PE potential or a Neutral that's in good shape, you won't get shocked - because the "ambient earth potential" is nearly the same as that mains working ground (neutral).

If OTOH you touch the live wire, while your body has some finite impedance against the GND/Earth potential, current will flow.

The redundant ground connections, i.e. the neutral wire in the mains and the ground rods and the underlying terrain, indeed comprise a mesh of interconnects. The general rule is: the denser the grounding mesh of wires and terrain, the safer for the people around. The point is to contain any serious level of fault current reaching the grounding mesh to just the closest neighborhood - so that the "earth mesh" does not exhibit dangerous voltage differentials between its nodes. It's down to Mr. Kirchhof.

If you'd disconnect your mains neutral (the wire coming from the neighborhood trafo) from the PE neutral at the base of the building, rest assured that there would be some parasitics visible on the oscilloscope. In our house, I can see the neutral wobble by several Volts - even between two "socket circuits" the neutral and the PE can dance around a little.

As @Harper has pointed out, there's no "absolute" voltage level - voltage is a difference of electric potentials between two points or circuit nodes. And no, in that sense your live wire is not the same as your return wire - exactly because there's a voltage source connected between them, applying a difference of potentials between these two "circuit nodes". It's the difference of potentials that makes current flow through a load (e.g. human body) that gets connected between the two circuit nodes.

Call it a mere circumstance that we call one of the nodes "neutral", and it happens to be connected to a broad equipotential mesh that we call ground or earth, and that serves as our universal "reference potential", assigned a voltage of 0V.

The safe way to get a severe shock is obviously by touching live and neutral directly :-) But even if you touch just the live node, there is still some residual current flowing through your body: due to some finite resistance of the soles of your shoes, and by capacitive AC coupling of your body to the surrounding earth mesh (I wouldn't dare to suggest that direct EM radiation into free space matters, at 50-60 Hz, considering the size of your body).

To us tiny mortal beings, the earth mesh is "everywhere around". Now imagine a static electric field in space. In our perspective and scale, any potential "different from earth" creates a locally focused gradient in the electric field, "spreading out" into the surrounding objects that are at earth potential. Given an AC voltage potential, different from the "ambient earth potential", your body connected to live, "drags a gradient towards it", in the surrounding electric field. The closer you are to surrounding objects at earth potential, the stronger the AC current flowing through your body...

Not sure how much of an exagerration this is = how much the capacitive / field-gradient coupling matters, compared to parasitic finite resistances in "insulators" such as the rubber soles of your shoes, the floor surface materials etc.

An electrostatic field simulation of a live wire and a human body - a simple illustration of the field gradients

The principal limitation of Agros is that it's "2D". Imagine the 2D picture extruded 1 meter into the third dimension (depth). Clearly this is not a perfect simulation of a human body - if the dummy figure was a proper 3D shape, the "gradient halo" around it would likely cling tighter to the figure, as it would have much less "mass" in the third dimension. Much less surface, actually. But, imperfect as it is, it does give you an idea.

The wire is only there as a small object for comparison. The fact that the dummy figure does not actually touch the wire in the picture means nothing - the "voltage on the edges", comprising the wire and the dummy figure shape, are configured (hardwired) to 240V. I guess you can imagine an actual electrical connection between them :-) I could've drawn a line from the wire to the figure, but in that pseudo-3D simulation, it would translate into a planar object, with an unrealistically large surface, which would give a very skewed simulation result (a much larger halo) - so I just skipped that connection.

\$\endgroup\$
2
  • \$\begingroup\$ This is very interesting. Others have pointed out that touching N will not shock either, as you say. So maybe my real question is: how does the electricity “know” it’s moved from the hot wire to the neutral? They’re identical wires, connected in a circuit, with electrons flowing through them. Yes, somewhere downstream the neutral is connected to PE. Ok. But isn’t the live wire connected to PE the same way? There’s the same continuity there. Can you explain what the distinction is, physically? Thank you. \$\endgroup\$
    – Skeptic
    Dec 4, 2021 at 7:35
  • \$\begingroup\$ @Skeptic I have enhanced my answer... \$\endgroup\$
    – frr
    Dec 4, 2021 at 9:46
0
\$\begingroup\$

Electricity Flows In Circuits

I think what's missing from your intuition is that electricity only flows significantly in circuits. Because the electrostatic force is incredibly strong, the number of electrons everywhere stays very close to the number of protons, and so the current leaving anything pretty much equals the current entering it.

The potential difference between HOT and your hand is not too significant all by itself. When someone rubs their feet on the floor and shocks you, the potential difference is much, much greater, but the shock is not an issue. Almost no current flows, because there is no circuit.

So, all the current that enters or leaves the supply transformer must be balanced. When you touch the hot wire, and there is a path between you and ground, the current flows from the supply transformer, to HOT, through you, though the ground, through the neutral-ground jumper, and through neutral back to the transformer. In this case, there is current flowing through your ground plate. It's the current that shocks you.

If there was no neutral-ground jumper, you might not get a shock at all. (Ground connects to a lot of things, though, so that is not a safe assumption!).

ALL the current that exits the supply transformer through HOT must return through neutral (or the other HOT, but that's a different story). The reason that no current flows though your ground plate under normal circumstances is simply that it only connects to the transformer though one path -- the neutral-ground jumper. There is no possible circuit that incorporates it.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.